U.S. Pat. No. 4,209,191 describes a tubular element for a riser pipe of an oil well. The elements are mutually coupled to form a pipe string, in particular a riser pipe string for underwater oil wells. The disclosed tubular elements comprise a male and a female coupling part for coupling to another element of the same form, which coupling parts are provided on respectively an outer periphery and inner periphery thereof with interlocking lugs. The lugs are provided in two series that are spaced apart in the axial direction of the element. The lugs in each series are aligned in the axial direction with corresponding lugs in the other series. U.S. Pat. No. 3,442,536A, US 2005/087985A1, U.S. Pat. Nos. 3,948,545A, 3,922,009A and 6,106,024A describe similar tubular elements.
An object of the present invention is to provide a tubular element, using which a fall pipe can be assembled which can be employed for relatively great water depths and which can be assembled more easily than the known fall pipe.
This object is achieved according to various embodiments by providing a tubular element. The element comprises a male and a female coupling part for coupling to another element of the same form, which coupling parts are provided on respectively an outer periphery and inner periphery thereof with an annular flange which is divided in peripheral direction into segments which leave recesses clear therebetween, wherein for coupling purposes segments of a male coupling part are placed through recesses of a female coupling part and are slid by rotation under segments of the female coupling part. By mutual coupling of a plurality of elements according to the invention a fall pipe is obtained which requires no cables to support the fall pipe. Each fall pipe element hangs were from the element above it, whereby a self-supporting fall pipe is obtained. For the same length the fall pipe according to the invention can hereby be given a lighter form, and in particular a stiffer form in axial direction, than the known fall pipe. A greater axial stiffness results in a higher natural frequency for axial stress waves, whereby resonance in the fall pipe caused by movements of the ship will occur less, or even not at all.
The assembly of the fall pipe according to the invention can hereby take place in highly motorized and automatic manner, whereby great lengths can be made and/or raised at a relatively high assembly or disassembly speed. Because the coupling of the elements of the fall pipe can have a greater load-bearing surface compared to the known fall pipe, a stronger fall pipe is provided which is therefore particularly suitable for greater water depths.
According to the invention, a tubular element is provided wherein the coupling parts are provided with at least two annular flanges which lie at a mutual axial distance and which are divided in peripheral direction into segments leaving recesses clear therebetween, whereby the segments of an upper annular flange are arranged offset relative to the segments of a lower annular flange. Because the present embodiment comprises at least two annular flanges, the load-bearing surface of the coupling is further increased, this further increasing the axial load-bearing capacity and therefore increasing the potential length of the self-supporting fall pipe. The allowable bending moment in the fall pipe also increases considerably.
The tubular element according to the invention is particularly suitable for assembling a fall pipe. The invention also provides a method for assembling such a fall pipe. The method according to the invention comprises of providing a plurality of tubular elements according to the invention, placing segments of a male coupling part of a first element through recesses of a female coupling part of a second element, and rotating the first element relative to the second element, this in a manner such that the segments of the male coupling part are slid under segments of the female coupling part. Because no additional coupling means, such as for instance cables, are required, a fall pipe is assembled in simple manner and within a shorter time than the known fall pipe. This is all the more advantageous since the fall pipe according to the invention is particularly suitable for application at greater water depths. Such an application requires the coupling of a large number of elements.
In the method for assembling a fall pipe according to the invention, the coupling parts are provided with at least two annular flanges which lie at a mutual axial distance and which are divided in peripheral direction into segments which leave recesses clear therebetween, and segments of a male coupling part of a first element are placed through recesses of a female coupling part of a second element, whereafter the first element is rotated relative to the second element, this in a manner such that the segments of the male coupling part are slid under segments of the female coupling part, and this rotation is repeated at least twice.
In the fall pipe according to the invention the segments of a coupling part lying at a mutual axial distance extend at a mutual distance. The load-bearing surface, and thereby the strength of the coupling of the elements, is hereby further increased.
The above described embodiment allows the coupling and uncoupling of the fall pipe elements to proceed in highly motorized and automatic manner, this enabling a high sustained assembly and disassembly speed of the fall pipe. In order to (un)couple the elements the fall pipe vessel is preferably provided with a device comprising at least a clamping device for a fall pipe, in which clamping device an element can be temporarily suspended. The clamping device is moreover rotatable around the axial axis of an element suspended therein, for instance by being provided with a turntable. In order to couple an element to an already partially assembled fall pipe, the element is suspended from its upper end in the clamping device and lowered in controlled manner. The lower end of the element is coupled into the upper end of the already partially assembled fall pipe by placing segments of a male coupling part of the element through recesses of a female coupling part of the uppermost element of the already partially assembled fall pipe (or vice versa), and rotating the element relative to the already partially assembled fall pipe, this in a manner such that the segments of the male coupling part are slid under segments of the female coupling part (or vice versa). The tubular element for coupling can be rotated at the top around the axis in the suspension by means of the turntable in the suspension. The clamping device is preferably also provided with a control system, with which the mass of the tubular element can be compensated so that either a light pulling force (uncoupling) or a light pressing force (coupling) can be exerted on the preceding tubular element by the element for coupling.
The extent to which the segments extend over the periphery of the annular flange or annular flanges can be varied within broad limits. The segments of an annular flange preferably extend over more than 25% of the periphery of the element, more preferably over more than 35% and most preferably over substantially 50% of the periphery of the element.
In a practical embodiment each annular flange comprises four segments distributed regularly in peripheral direction. This results in a good compromise between a good locking and user-friendliness during assembly of the fall pipe.
The segments can have a varying form. The segments preferably comprise rounded portions, whereby the coupling of elements is further simplified. Rounded portions are also understood to mean chamfered portions.
In an advantageous embodiment the segments are positioned and/or formed such that mutually coupled elements can move relative to each other. Assembly of the fall pipe hereby proceeds more easily, since small relative movements of the two tubular parts for (un)coupling (fall pipe and part for (un)coupling) are then possible. The loads possibly occurring during use of the fall pipe, for instance as a result of currents, impacts and the assembly of the fall pipe, are hereby at least partially absorbed. A certain flexibility of the fall pipe (in the XZ and YZ plane) moreover has advantages. The Z-direction corresponds here to the axial direction of the fall pipe, while the (X,Y)-plane extends perpendicularly of the Z-direction.
The coupling parts and/or the annular flanges can be manufactured from various materials. The coupling parts and/or the annular flanges are preferably manufactured substantially from a steel alloy, an aluminium alloy and/or a fibre-reinforced plastic. These materials combine an adequate strength and processability with a low weight. The use of materials with a high strength/mass ratio is particularly appropriate when working at great depths, so that the load on the installation from which the fall pipe is hanging is not allowed to become unnecessarily great. This also applies for the power to be installed for the purpose of raising the fall pipe in a relatively short period of time.
Fibre-reinforced plastics comprise reinforcing fibres incorporated in a matrix of a plastic. The matrix of the fibre-reinforced plastic can comprise a thermoplast or a thermoset. The reinforcing fibres preferably comprise glass fibres and/or carbon fibres. In a preferred embodiment the fibre-reinforced plastic comprises carbon fibres in an epoxy matrix. Such a fibre-reinforced plastic combines an adequate strength with a low weight.
A particularly suitable aluminium alloy is the 1980 T1 alloy. Such an alloy is not only found to possess a strength such that a fall pipe of relatively great length can be provided, but it also combines a good corrosion-resistance with a good processability and weldability.
A particularly suitable embodiment comprises a tubular element which is manufactured by means of an extrusion process and thereby has very high and uniform mechanical characteristics, particularly in the axial direction, and wherein the male and female coupling parts are manufactured from forgings, whereby they have reliable mechanical characteristics.
The coupling parts can be connected in various ways to an element. It is thus possible to connect at least a coupling part integrally to the element, for instance by integral casting thereof with the element. A simple and strong element is hereby provided, wherein no additional connecting means are required. It has however been found advantageous to embody the coupling part separately and connect it to the element by making use of so-called friction stir welding. An accurate and strong connection of the coupling part to the element is obtained by making use of friction stir welding. It has been found that the strength of the material in the weld and in the zone affected by the welding heat is hardly less than that of the parent material.
Elements of the fall pipe may displace, in particular rotate, relative to each other due to for instance the depositing of material, currents and movements of the vessel or possible contact of the fall pipe with the bottom, whereby there can occur the risk of uncoupling of adjoining elements. In a further preferred embodiment the fall pipe according to the invention therefore comprises a locking or securing element for mutual locking of a male coupling part of an element and a female coupling part of an adjoining element. This reduces the risk of adjoining segments being able to displace relative to each other, particularly during use, and thereby possibly being uncoupled. The locking element particularly comprises a ring provided with projections which extend at a mutual distance and which can be placed between the segments of an annular flange. Such a locking element is simple and reliable. In a corresponding method for assembling a fall pipe the method is characterized by locking the first and the second element against relative rotation, and thereby against uncoupling, after mutual coupling of these elements.
In another preferred embodiment the above stated lock, or optionally another element, also provides for sealing on the outer side of the space between the female and the male coupling elements. Particles floating in the sea could after all settle in this space and thereby jam the coupling, or a water flow could occur from the sea through the coupling to the zone in the fall pipe, and this is not desirable.
The sealing element is preferably manufactured substantially from a flexible material such as a rubber or a polyolefin such as polyethylene. The sealing element is particularly embodied integrally with the locking element, which makes the fall pipe simple and inexpensive and also limits additional operations during assembly of the fall pipe.
In another preferred embodiment the tubular element comprises a sealing element for mutual liquid-tight coupling of two adjoining elements to the inner wall of the fall pipe. This improves the operation of the fall pipe. Possible feed of water into the fall pipe should preferably take place in controlled manner.
In a preferred embodiment the fall pipe is provided on the inner side with a wear lining which protects the load-bearing structure. This lining is preferably repairable and/or replaceable. A suitable material for the wear lining comprises polyurethane (PU).
In a further preferred embodiment use is made of the above stated wear lining to realize the above-mentioned seal on the inner wall of the fall pipe, for instance in the form of a labyrinth seal by finishing the lining at 45° on one side of the tubular element and at the corresponding angle of 45° at the other end, thereby creating an overlap and a simplified form of a labyrinth seal.
Although a fall pipe can be assembled of any desired length using the element according to the invention, the element is particularly suitable for assembling a fall pipe with a length of more than 1000 meters, more preferably of more than 1250 meters and most preferably of more than 1500 meters. Such heretofore unachievable lengths allow material to be deposited at great water depths, and this with sufficient precision.